A Quick Overview Of How To Say Ad Infinitum
'Ad infinitum' is Latin for 'to infinity'. We can't be sure of how Latin was originally pronounced but from the 4th century, Latin has been used by the Catholic church, and the pronunciation from this time on has remained stable up to modern times. Using this pronunciation, 'ad infinitum' sounds like 'ad in/fi/NEE/toom'.
The 'new' or 'modern' way of saying 'ad infinitum' follows modern pronunciation rules, and has become correct by usage. It is: 'ad in/fi/NI/t[e]m', with the capital 'I' sounding like 'eye', and a neutral vowel on the 'u'. (A neutral vowel makes an 'uh' sound like the 'a' in 'China', and the 'e' in 'the'). To put it simply, just pronounce the '-itum' like the word 'item'. This is the most popular pronunciation today, and the one you are likely to hear in the media.
If you want to sound old-worldly and scholarly, the old pronunciation is the way to go. Otherwise, use the modern pronunciation.